Summary from MetaCyc:
Diacetyl is an important flavor component in many fermented foods and beverages, including dairy and dairy-like products, beer and wine [Bartowsky04]. It imparts a buttery aroma and flavour, and has an important stylistic role in wine. In beer, diacetyl is considered a spoilage factor. During beer maturation, 2-acetolactate is slowly converted to diacetyl by a chemical, non-enzymatic process, and the diacetyl is converted to acetoin by diacetyl reductase of the yeast cells. for this purpose, purified enzyme is sometimes added to the wort [Diderichsen90].